Q=) K. MITSÜKÜRI; 



meiitioiied l)y Balfouu. The two sides of tliis bay, which it will be 

 remembered are portions of the edges of the blastoderm, come to lie 

 close too-ether on the volk beneath the tail of the embryo. For a 

 little time they remain unfused, and the yolk is still freely exposed 

 between them in a linear streak.* This slit, wliich is bounded b}'' 

 the edges of the non-embryonic part of the l)]astoderm of the two 

 sides, is a part of the blastopore, and is continuous, passing along the 

 hinder side of what will be the umbilical stalk, with the portion of 

 the blastopore leading into the hind gut and extending along the 

 ventral side of the tail. This last portion is * * * continuous 

 with a dorsal portion which leads through the medullary j)late into 

 the medullary canal. 



" The last part of the blastopore to be mentioned is the so-called 

 yolk-blastopore, described by Balfouk in the ' Elasmobranch Fishes ' 

 p. 81 (Mem. Edition, vol. iii, p. 296), and in the ' Comparative 

 Embryology,' 1st ed., ch. iii, p. 52. The lips of this portion are 

 continuous with the lips last mentioned as running back on the yolk 

 parallel to one another and ventral to the tail of the embryo. 



'' To recapitulate : the blastopore of Elasmobranchii is at the 

 present stage — i.e. the stage inunediately before closing — an elongated 

 narrow slit, slightly dilated in front, where it lies on the floor of the 

 medullary canal (Fig. 3) and more dilated behind (Balfour's yolk- 

 blastopore, ' Comparative Embryology, vol. ii, ch. iii. Fig. 30^). 

 Between these two limits it takes the course of a reversed letter S, as 

 shown in tlie adjoining woodcut,f where its lips are represented 

 unfused. 



" The anterior part, a ]>, perforates the floor of the medullary 

 canal, and is dorsal; this is continuous round the end of the tail, 



* See ScHWARz's Fig. 16. or Ziegler Fig. 24, I. 

 t See Woodcut XTI on the next pnge. 



